Instantaneous creep in face-centered cubic metals at ultralow strain rates by a high-resolution strain measurement

Instantaneous creep in face-centered cubic metals, 5N Al (99.999%), 2N Al (99%) and 4N Cu (99.99%) with different grain sizes, was firstly investigated by sudden stress-change experiments at ultralow strain rates dot \varepsilon ≤ 10^-10 s^-1 and temperature T < 0.32 T _m. The experimental results indicate that the observed instantaneous creep is strongly dependent on grain size, the concentration of impurity, and stacking fault energy. Creep in high-purity aluminum, 5N Al, with a very large grain size, d _g > 1600 μm, shows non-viscous behavior, and is controlled by the recovery of dislocations in the boundary of dislocation cells. On the other hand, for 5N Al with a small grain size, d _g=30 μm, and low-purity aluminum, 2N Al, with d _g= 25 μm, creep shows viscous behavior and may be related to 'low temperature grain boundary sliding'. For high-purity copper, 4N Cu, with d _g= 40 μm and lower stacking fault energy, creep shows a non-viscous behavior, and is controlled by the recovery process of dislocations. For all of the samples, creep shows anelastic behavior.